Before I can publish any of the letters that I copied at the Bentley Library I must first get written permission from both the library and the copyright owner. This is a standard requirement of the library and applies to all of their collections. Today I received permission from the library with the caveat that it is my responsibility "to determine what constitutes a fair and reasonable effort to abide by copyright law."
Since my grandmother was the donor of the collection her heirs would be the owners - or would they?
I've done some reading and I think the copyright was/is held by each individual author, rather than the recipient. Since I do not believe that any of the letters meet the legal definition of published, I believe that per this table all of the letters whose authors died prior to 1938 are safely in the public domain.
For any authors that died after 1937 I would have to track down their heirs and get written permission from each. In some cases this would be a huge task, in other cases it might be doable. If I can't obtain written permission I believe that I can still use small parts of letters as "fair use." For example I might use a sentence or two from a letter that gives the date and place of death of a family member.
For authors that are still living I would need their written permission. Even though I could probably use parts of these letters as fair use, I will ask permission first. If someone is still living and doesn't want me to use anything they have written I will not.
There are dozens of letters that I believe are now in the public domain and plan to begin with them.
If you have more knowledge of how copyright law would apply to these letters and/or the genealogy sheets and notes that make up the collection I would appreciate your input.